1. Field of the Invention
The present invention relates to a spectrum diffusion signal analyzer, which is used for measuring an error of a carrier wave frequency with respect to a reference frequency or a signal period timing in the field of a spectrum diffusion communication, and for demodulating the spectrum diffusion signal.
2. Description of the Related Art
With respect to the wireless communication of the CDMA (Code Division Multiple Access), if a spectrum diffusion signal is received, it is necessary to synchronize the diffusion code such as PN code, Walsh code, or Gold code, of the received signal with a diffusion code signal of a local station. In order to make the synchronization, there is a method of searching a correlation between the inputted signal and the diffusion code signal, and of searching the point wherein the correlation value has the maximum value with respect to a time axis.
On the other hand, with respect to the wireless communication, there is an error in the received carrier wave frequency with respect to a reference carrier wave frequency. The error is caused by the Doppler shift or a frequency error of a reference frequency source provided in a transmitter. There is a possibility that a measuring device cannot detect an error in setting a center frequency, or cannot detect a reference frequency exactly.
If there is a carrier wave frequency error, it is difficult to make synchronization with respect to a time axis, since the peak of the correlation value does not appear. Therefore, there is a method including the steps of dividing a frequency range, and searching a synchronization point by sequentially changing a received carrier wave frequency. If a carrier wave frequency is shifted by xcex94xcfx89, a frequency of a complex signal Z(i) obtained by an orthogonal transformation (i is the number of samples and represents a time) is corrected, and therefore, it is expressed by Z(i)xc3x97exp(xe2x88x92j xcex94xcfx89i). Then, a squared absolute value of the product of the corrected signal and the complex-conjugate R* of the diffusion code signal R, i.e. a correlation, is calculated.
|xcexa3i=0L[Z(i)xc3x97exp(xe2x88x92jxcex94xcfx89i)]xc3x97R*(i)|2xe2x80x83xe2x80x83(1)
The time shift of the squared absolute value of the correlation value, i.e. the peak of the correlation curve is the synchronous position. The correlation curve is given as follows:
C(m)=|xcexa3i=0L[Z(m+i)xc3x97exp(xe2x88x92jxcex94xcfx89xc2x7(m+i)]xc3x97R*(i)|2
The frequency-shifted correlation curve is represented as follows:
Cxe2x88x92(N/2xe2x88x921)(m)=|xcexa3i=0L[Z(m+i)xc3x97exp(xe2x88x92jxc2x7(xe2x88x92(N/2xe2x88x921))xc2x7xcex94xcfx89xc2x7(m+i)]xc3x97R*(i)|2
Cxe2x88x921(m)=|xcexa3i=0L[Z(m+i)xc3x97exp(xe2x88x92jxc2x7xe2x88x92xcex94xcfx89xc2x7(m+i))]xc3x97R*(i)|2
C0(m)=|xcexa3i=0L[Z(m+i)xc3x97exp(xe2x88x92jxc2x7(m+i))]xc3x97R*(i)|2
C1(m)=|xcexa3i=0L[Z(m+i)xc3x97exp(xe2x88x92jxc2x7xcex94xcfx89xc2x7(m+i)]xc3x97R*(i)|2
xe2x80x83C2(m)=|xcexa3i=0L[Z(m+i)xc3x97exp(xe2x88x92jxc2x72xcex94xcfx89xc2x7(m+i))]xc3x97R*(i)|2
CN/2(m)=|xcexa3i=0L[Z(m+i)xc3x97exp(xe2x88x92jxc2x7(N/2))xc2x7xcex94xcfx89xc2x7(m+i)]xc3x97R*(i)|2
Here, N is one period of the diffusion codes (code length).
Because of the frequency error, the frequency having the maximum peak of the correlation curve peak is searched by detecting the correlation curves by shifting the frequency range xcex94xcfx89 by xcex94xcfx89.
The above is explained in detail, for example, in the xe2x80x9cSpectrum Diffusion Communication Systemxe2x80x9d, pages 333-337, written by Yokoyama, published by Science Technology Publishers.
However, a time synchronization is performed by sequentially changing a frequency range, and by shifting the diffusion code a half chip by a half chip within the frequency range. Then, if a peak value greater than a threshold value cannot be obtained, the same steps are repeated with a shifting of the frequency range. Therefore, a hardware for sequentially changing the frequency is needed. Or alternatively, if it is performed by a software, a time for changing a frequency of a signal is necessary. Namely, there is a problem that the hardware scale becomes greater, or a problem that a processing time becomes longer.
Accordingly, an object of the present invention is to substantially eliminate defects and drawbacks encountered in the prior art and to provide a spectrum diffusion analyzer, which can detect a displacement of a carrier wave frequency of a received signal with respect to a reference carrier wave frequency, and a timing drift of a diffusion signal without changing a frequency.
According to the present invention, the above mentioned object can be achieved by a spectrum diffusion analyzer including: a device for orthogonal-transforming inputted spectrum diffusion signal; a device for calculating a product of the orthogonal-transformed signal and a complex-conjugate of a diffusion code signal; a device for discrete-Fourier-transforming data series of the calculated product; and a device for calculating a squared absolute value of each Fourier-transformed coefficient.
According to the present invention, an inputted spectrum diffusion signal is orthogonal-transformed, and a product of the orthogonal-transformed signal and a complex-conjugate of a diffusion code signal is calculated. Then, data series of the calculated product is discrete-Fourier-transformed, and a squared absolute value of each Fourier-transformed coefficient is calculated.
The spectrum diffusion analyzer may further include: a peak searching device for searching a displacement of said diffusion code signal with respect to a reference time when said each squared value has a maximum value within one period of the diffusion code signal, and for searching a frequency of the corresponding coefficient.
According to the present invention, a displacement of said diffusion code signal with respect to a reference time when said each squared value has a maximum value within one period of the diffusion code signal, and a frequency of the corresponding coefficient as a carrier wave frequency error are searched.
Namely, with respect to the above formula (1), the transformation of the product is performed within the bracketed area represented by the symbol xcexa3. The formula (1) is represented by the following formula (2), since m is constant within the absolute value symbols.   "AutoLeftMatch"                                                                                          C                  ⁡                                      (                    m                    )                                                  =                                  xe2x80x83                                ⁢                                  "LeftBracketingBar"                                                            ∑                                              i                        =                        0                                            L                                        ⁢                                          xe2x80x83                                        ⁢                                                                  [                                                                              Z                            ⁡                                                          (                                                              m                                +                                i                                                            )                                                                                xc3x97                                                                                    R                              *                                                        ⁡                                                          (                              i                              )                                                                                                      ]                                            xc3x97                                              exp                        ⁡                                                  (                                                                                                                    -                                j                                                            ·                              Δ                                                        ⁢                                                          xe2x80x83                                                        ⁢                                                          ω                              ·                              i                                                                                )                                                                    xc3x97                                                        "AutoRightMatch"                                                                                                                                              xe2x80x83                                    ⁢                                      exp                    ⁡                                          (                                                                                                    -                            j                                                    ·                          Δ                                                ⁢                                                  xe2x80x83                                                ⁢                                                  ω                          ·                          m                                                                    )                                                        "RightBracketingBar"                                2                                                                                        =                                  xe2x80x83                                ⁢                                                      "LeftBracketingBar"                                                                  ∑                                                  i                          =                          0                                                L                                            ⁢                                              xe2x80x83                                            ⁢                                              {                                                                              [                                                                                          Z                                ⁡                                                                  (                                                                      m                                    +                                    i                                                                    )                                                                                            xc3x97                                                                                                R                                  *                                                                ⁡                                                                  (                                  i                                  )                                                                                                                      ]                                                    xc3x97                                                      exp                            ⁡                                                          (                                                                                                                                    -                                    j                                                                    ·                                  Δ                                                                ⁢                                                                  xe2x80x83                                                                ⁢                                                                  ω                                  ·                                  i                                                                                            )                                                                                                      }                                                              "RightBracketingBar"                                    2                                                                                          (          2          )                    
Formula (2) is a formula for discrete-Fourier-transforming (Z(m+i)xc3x97R*). Therefore, according to the present invention, the product of the inputted orthogonal-transformed signal Z(m+i) and the complex-conjugate R* of the diffusion code series are calculated with respect to each sample, and then, the data series resulted from the calculation are discrete-Fourier-transformed. Finally, the squared absolute value of each coefficient obtained from the discrete-Fourier-transformation is a correlation value with respect to m. If the discrete-Fourier-transformation is performed with respect to each frequency, Cf(m) is calculated. By using the FFT (Fast-Fourier-Transformation) with respect to the discrete-Fourier-transformation, Cf(m) is calculated with respect to each discrete-frequency. If the same calculation is performed with respect to m, m+1, m+2, . . . , the two-dimensional correlation data with respect to the discrete-frequency (frequency error) and the discrete-time (timing displacement) are obtained.
According to the present invention with respect to the spectrum diffusion signal analyzer, the peak searching device is a display device, which includes one of orthogonal axes as a frequency axis and the other as a time axis, for displaying the squared value.
Furthermore, according to the present invention, the display means three-dimensionally displays a level of the squared value by using another axis, which is perpendicular to the both of the orthogonal axes.
According to the present invention, the display means displays a level of the squared value by using a level of brightness, or the display means displays a level of the squared value by using colors, each of which respectively corresponds to each level of the squared value.
According to another aspect of the present invention, a method of analyzing spectrum diffusion signal includes an AD transformation step of transforming inputted spectrum diffusion signal to digital data series; an orthogonal-transformation step of orthogonal-transforming the digital data series, and temporally storing the orthogonal-transformed data series; a multiplication step of complex-multiplying each sample having the orthogonal-transformed data series and the complex conjugate of the diffusion code signal; a Fourier-transformation step of discrete-Fourier-transforming samples corresponding to a period of the diffusion code of the complex-multiplied sample; a square calculation step of calculating squared absolute values with respect to each frequency component of the discrete-Fourier-transformed samples; a step of repeating the multiplication step, the Fourier-transformation step, and the square calculation step with sequentially shifting a relative phase between the orthogonal-transformed data series and the diffusion code signal within a chip period until a sum of the shifted relative phases arrives at one period of the diffusion code; and a search step of searching a timing drift of the diffusion code with respect to that of inputted signal in accordance with a calculated result of said square calculation step, and of searching a carrier wave frequency error of the inputted signal.
According to the present invention with respect to this method, a sampling frequency used at the AD transformation step is J times as common as a chip frequency of the diffusion code signal, wherein J is an integer, and the said multiplication step is performed at every J samples.
Furthermore, according to the present invention, the diffusion code signal includes a series of binary data, and at the multiplication step, adding process is performed with or without transforming a code of the orthogonal-transformed data in accordance with a value of the binary data of the diffusion code signal.
According to the present invention, the search step includes the steps of displaying the squared value with using one of orthogonal axes as a frequency axis and the other as a time axis, searching a maximum peak of the squared value, and searching the corresponding frequency and time.
Furthermore, according to the present invention, a level of the squared value is displayed by using a level of brightness, or by using colors, each of which respectively corresponds to each level of the squared value.
The nature, utility, and further features of this invention will be more clearly apparent from the following detailed description with respect to preferred embodiments of the invention when read in conjunction with the accompanying drawings briefly described below.